Detecting values in GET Http header

Hi. I have a small but annoying problem.
I'm using HTML with GET. In the page there are:
1.Combo box in which : 1.1. a value is always selected
1.2 'On change' event causes the form to
submit
2. Some submit buttons
The problem is:
After the form is posted to the server, I do not know if it was submitted
because that the user changed the value in the combo box, or beacuse he clicked one of the submit buttons.
( the GET ALWAYS contains the value selected in the selected box)
Possible Solutions:
1. On combo box's ON change event -> submit a hidden button,
and then check on the server if this button name was sent to the server.
( but I was able to do it with type="Submit" NOT with type="button"
So, as a result the 'hidden' button appears on the page!!!
2. Using CSS to hide the button doesn't work as well.